0,0,0 will be the corner of the texture. which corner depends on which side of 0,0,0 you are on and how you are looking at the world and it doesn’t really matter. If you want to debug it, Clamp the coordinates from 0 to 1. That will show you a single texture in PositiveX and Positive Y.
To make i the center of the texture on 0,0,0, simply add 0.5 after any scaling.